The invention relates to a method and a system for liquefying a gas, in particular a natural gas, by cooling it under pressure in stages, wherein the natural gas is cooled by means of at least two refrigerants. One refrigerant circulates in a closed refrigeration circuit and the othr refrigerant circulating in another closed refrigeration circuit, wherein the compositions of the two refrigerants differ from each other.
Various embodiments of the above method and system are known, for example, in British patent specification No. 895,094, published on May 2, 1962, British patent specification No. 1,572,898 published on Aug. 6, 1980, and British patent specification No. 1,291,467 published on Oct. 4, 1972.
In these known methods and systems various kinds of refrigerants are used, for example, a single component refrigerant in a closed refrigeration circuit and a so-called multi-component or mixed refrigerant in another closed refrigeration circuit, or a multi-component refrigerant in a closed refrigeration circuit and another multi-component refrigerant in another closed refrigeration circuit.
For example, in British patent specification No. 1,291,467 an embodiment of the above-mentioned method is described, wherein a refrigerant circulating in a closed refrigeration circuit is propane and another refrigerant circulating in another closed refrigeration circuit is a multi-component refrigerant comprising nitrogen, methane, ethane and propane.
In liquefaction methods and systems of the above known kind, in each closed refrigeration circuit the refrigerant is normally compressed in at least one compressor, cooled in order to remove the compression heat and further cooled in order to liquefy the refrigerant. Then the liquid refrigerant is expanded in one or more stages and evaporated to produce refrigeration, whereafter the refrigerant is returned to the compressor to repeat the cycle.
The cooling of the refrigerant after compression is often carried out by means of water supplied from a river or from the sea and returned to the river or sea after the cooling step. This has the advantage that the temperature of the cooling water is reasonably constant. It has the disadvantage, however, that large capital investments are necessary for pipelines, pumps and other equipment for supplying the water to the liquefaction system and for returning the water to the river or the sea. A substantial reduction of the necessary capital expenditure is possible by carrying out the cooling step by means of ambient air.
A further reduction of capital expenditure can be obtained by driving the compressors by gas turbines, instead of by steam turbines, since in that case large capital investments in a steam generating plant are unnecessary.
However, when driving a compressor of a closed refrigeration circuit by means of a gas turbine and when cooling the refrigerant after compression by means of air, the following problems occur. In the first place, a gas turbine can deliver more power at lower ambient temperatures than at higher ambient temperatures. Moreover, when the refrigerant after compression is cooled by ambient air and is condensed against ambient air, this has the result that at lower temperatures of the ambient air, the compressor in the refrigeration circuit requires less power than at higher temperatures of the ambient air. The result is that a lower ambient temperatures the gas turbine driving the compressor in the refrigeration circuit runs at part of its design load and thus not at optimal efficiency, because the process and/or equipment constraints do not allow full shifting of available power.